The general calculus is that an interceptor costs as much as the missile it intercepts, so defenses are only effective against an adversary with much less resources. Hence Israel can defend against Hamas/Hezbollah, and the US can defend against North Korea, but Israel struggles against Iran and the US doesn't even try to defend against China/Russia.

China obviously has a lot more resources than Taiwan, but then you have a concentration effect where an attacker can focus their resources on a single target, but a much more resourced defender can't necessarily afford to defend that target. We saw that play out with the UK's nuclear deterrent strategy in the cold war, where they focused on overwhelming Moscow's defenses, and were (probably) able to do it despite the USSR being so much bigger.

TW missiles can't "concentrate" because TW geography = all missile flight paths travels through boost, midcourse and terminal interceptors gauntlet along PRC easter theatre command which probably has the densest IADS in the world. PRC has like 3-4x more interceptors in eastern theater (8-12x more total) than TW has missiles. That's just land based, there's also 1000s of naval picket interceptors. Imagine if all of US patriot batteries in Florida, multiplied by 3, then asking what Cuba can do to saturate. Then add in USN DDGs and the answer is realistically nothing, because the industrial math is brutally lopsided. That's assuming TW gets to coordinate salvo their entire inventory, realistically most TELs would be glassed first, every part of TW is withing 5-7 min of PRC missiles and mlrs, less if fired from strait or loitering munitions, i.e. basically faster than abbreviated TEL setup cycle TW has for their tunnel to launch strike complex.